t refers to the composition of metal structures with the same chemical properties, crystal structure, and physical properties, including solid solutions, metal compounds, pure substances, etc.
Grain size
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[001] Measurement of the average grain size of metals... GB 6394-2002
Grain size
[010] Measurement of grain size of cast aluminum copper alloys... GB 10852-89
[019] Measurement of the average grain size of pearlite... GB 6394-2002
[062] Evaluation of the average grain size of metals... ASTM E112
[074] Black and white phase area and grain size evaluation... BW 2003-01
[149] Determination of the average grain size of color sample images... GB 6394-2002
Metal structure, the internal structure of metals and alloys observed by metallographic techniques. It is divided into 1. macrostructure, 2. microstructure.
Microstructure
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Metallography is the science that studies the internal structure of metals or alloys. Not only that, but also study the effects on the internal structure of metals and alloys when external conditions and internal factors change.
The so-called external conditions refer to temperature, processing deformation, injection conditions, etc. The so-called intrinsic factors mainly refer to the chemical composition of metals or alloys. Metal structure reflects the specific morphology of metal structure phases such as martensite, austenite, ferrite, pearlite, etc.
Austenite
Austenite
1. Austenite - A solid solution in which carbon and alloying elements are dissolved in γ-Fe and still maintain the face-centered cubic lattice of γ-Fe. The grain boundaries are relatively linear and regular polygonal, and the retained austenite in hardened steel is distributed in the gaps between martensite.
2. Ferrite
3. 2. Ferrite - A solid solution in which carbon and alloying elements are dissolved in α-Fe. The slowly cooled ferrite in hypoeutectoid steel is blocky, and when the carbon content approaches the eutectoid composition, ferrite precipitates along the grain boundaries.
4. Ferrite
5. Cementite
6. 3. Cementite - a compound formed by carbon and iron. In liquid iron-carbon alloys, the first cementite to crystallize alone (primary cementite) is block-shaped without corners, while eutectic cementite is skeletal. When hypereutectoid steel is cooled, carbides precipitated along the acm lines (secondary cementite) form a network, and eutectoid cementite becomes flaky. When iron-carbon alloys are cooled below ar1, cementite (tertiary cementite) precipitates from ferrite and appears as discontinuous flakes on the secondary cementite or at the grain boundaries.
7. Pearlite
8. 4. Pearlite - a mechanical mixture of ferrite and cementite formed by the eutectoid reaction in iron-carbon alloys.
9. The distance between flakes of pearlite depends on the degree of undercooling during austenite decomposition. The greater the degree of undercooling, the smaller the distance between the pearlite sheets formed. In the pearlite lamellar layer formed at a1-650 ° C, parallel wide ferrite layers and thin lamellar cementite can be distinguished under a metallurgical microscope at a magnification of 400 times or more, and they are called coarse pearlite and lamellar pearlite. Pearlite. When pearlite formed at 650-600 ° C is magnified at a magnification of 500 times under a metallurgical microscope, only black lines are visible in the cementite of the pearlite, and the lamellar structure that cannot be identified unless magnified at a magnification of 1000 times is called troxtenite. When pearlite formed at 600-550 ° C is magnified at a magnification of 500 times under a metallurgical microscope, only black pellet-like structures are visible, and they can only be identified under an electron microscope. They are called troubadour bodies.
10. Upper bainite
11. 5. Upper bainite - a mixture of supersaturated acicular ferrite and cementite, with cementite present between the ferrite needles. A phase transformation product of supercooled austenite at medium temperatures (about 350-550°C), its typical shape is a bundle of nearly parallel ferrite laths with a misorientation of 6-8od, distributed along the plate between each lath. Rods or pieces of carbides aligned along the long axis. Usually, upper bainite is feather-like, with the grain boundaries as the axis of symmetry. The feathers can be symmetric or asymmetric, as they have different orientations. They can be needle-like or point-like, block-like. In high carbon and high alloy steels, the feathers of the needles are not clearly visible, but in medium carbon and medium alloy steels, the feathers of the needles are more distinct and the needles are very distinct. . During transformation, upper bainite first forms at the grain boundaries and grows intragranularly without crossing the grains.
12. Lower bainite
13. 6. Lower bainite - Same as above, but cementite is present among the ferrite needles. A transformation product of supercooled austenite at 350°C~ms. The typical shape is a biconvex lenticular ferrite containing supersaturated carbon, inside which small carbide flakes are distributed in one direction arrangement, and in the crystal, they are needle-like, and the needles can be connected without crossing. Unlike tempered martensite, martensite is divided into layers, but the lower bainite has the same color. The carbide spots in the lower bainite are thicker than tempered martensite, and are more easily corroded and do not turn black. It is easily eroded. High carbon high alloy steels have a higher carbide dispersion than low carbon low alloy steels, and the needles are thinner than those in low carbon low alloy steels.
14. Granular bainite
15. 7. Granular bainite - a complex structure with many small islands distributed within large ferrite or band-like ferrite. The transformation product of supercooled austenite at the top of the bainite transformation temperature zone. When first formed, it consists of blocky ferrite and small islands of carbon-rich austenite, and sometimes all the carbon-rich austenite remains as retained austenite during the subsequent cooling process. A mixture of ferrite and cementite (pearlite or bainite), most likely to partially transform into martensite and partially retain to form a two-phase mixture called m-a structure.
16. Carbide-free bainite
17. 8. Carbide-free bainite - A structure consisting of a single phase of lath-like ferrite, also called ferrite-bainite. Its formation temperature is at the top of the bainite transformation temperature zone. Between the laths of ferrite is carbon-rich austenite, which undergoes a similar transformation during the subsequent cooling process. Carbide-free bainite generally occurs in low carbon steels and is also prone to form in steels with high silicon and aluminum content.
